1. Field of the Invention
The present invention relates to a switch for controlling a flow of a signal in a high frequency band wireless communication system or a radio frequency system, and more particularly, to a micro-electro mechanical systems (MEMS) switch driven by an electrostatic force.
2. Description of the Related Art
A field effect transistor (FET) and a pin diode are generally used as a switching element to control a flow of a signal in a high frequency band communication system. However, such a semiconductor switch has high insertion loss and low signal isolation loss although the semiconductor switch has a high degree of integration. Also, the semiconductor switch is a non-linear element that causes signal distortion. In order to overcome such drawbacks of the semiconductor switch, a micro-electro mechanical systems (MEMS) switch was introduced.
The MEMS switch generally includes a moving part that relatively moves with respect to a fixed substrate and a driving part for driving the moving part. The driving part includes two electrodes facing one another. The moving part is driven by electrostatic force generated by voltage supplied from the electrodes of the driving part. That is, the moving part moves horizontally or vertically to the substrate, or rotates about the substrate at a predetermined angle.
FIG. 1A is a plan view of a conventional MEMS switch having a cantilever structure.
Referring to FIG. 1A, the conventional MEMS switch having a cantilever structure includes a substrate (not shown) having a bottom electrode 2, a signal line 3 and a supporting member (not shown), and a cantilever arm 5 having an one end fixed at the substrate to be spaced apart from the bottom electrode 2 and the signal line 3 by a predetermined distance. A top electrode 6 is formed on the cantilever arm 5 and a contact member 7 connecting the signal line 3 is formed on a bottom of other end of the cantilever arm 5. A middle portion of the cantilever arm 5 and the top electrode 6 is formed to be narrower than other portions so that the other end of the cantilever arm 5 has a predetermined level of elastic force. As shown in FIG. 1A, the conventional MEMS switch includes a capacitor structure portion 8 formed of a plurality of small rectangles which are holes to eliminate a sacrificial layer that was formed on a bottom of the cantilever arm 5.
FIG. 1B is a cross-sectional view of FIG. 1 taken along a line A1-A1.
As shown in FIG. 1B, the cantilever arm 5 is apart from the bottom electrode and the signal line 3 at a predetermined gap because the thickness of the supporting member 4 formed on a left side of the substrate 1 is thicker than the bottom electrode 2 and the signal line 3. The contact member 7 is formed on the bottom of other end of the cantilever arm 5.
When a predetermined level of voltage is applied to the top electrode 6 and the bottom electrode 2, the electrostatic force is generated from the capacitor structure portion 8 formed by the overlapping of the top electrode 6 and the bottom electrode 2. Then, the electrostatic force bends the cantilever arm 5 in a bottom direction. Therefore, the contact member 7 connects the signal lines 3 to perform a switching operation. Such a conventional MEMS switch having the cantilever arm structure is disclosed in U.S. Pat. No. 5,578,976 (Nov. 26, 1996).
FIGS. 2A and 2B are cross-sectional views of the conventional MEMS switch shown in FIG. 1A taken along the line A2-A2 for describing operations of the conventional MEMS switch having the cantilever arm structure.
FIG. 2A shows the cantilever arm 5 with the contact member 7 of the conventional MEMS switch, which is operated in a normal state. That is, the cantilever arm 5 maintains to be parallel from the signal line 3 while moving upwardly and downwardly as shown in FIG. 2A. Although the signal line 3 connected to an input unit (not shown) and an output unit (not shown) and the contact member 7 are disposed to be parallel one another, the only one end of the cantilever arm 5 is supported by the supporting member 4 as shown in FIGS. 1A and 1B. Therefore, the cantilever arm 5 or the top electrode 6 may be modified due to thermal expansion while manufacturing the MEMS switch or operating the MEMS switch.
FIG. 2B shows the cantilever arm 5 with the contact member 7 of the conventional MEMS switch, which is modified due to the thermal expansion. As shown in FIG. 2B, other end of the cantilever arm 5 is not parallel to the signal lines 3 while the cantilever arm 5 moves upwardly and downwardly. Therefore, the cantilever arm 5 is unstably operated. Such an unstable operation of the cantilever arm 5 causes the loose contact that increases contact resistance of the signal line 3 and decreases the reliability by making the flow of the signal to be unstable.
FIG. 3 is a plan view of a conventional MEMS switch having a membrane structure.
Referring to FIG. 3, the conventional MEMS switch having the membrane structure includes a substrate 12 having a supporting member 24, a bottom electrode 14 and a signal line 18 having an opened portion, and a moving plate 20 disposed to be separated from the substrate at a predetermined gap and supported by the supporting member 24. The moving plate 20 includes a top electrode 16 and is supported by the supporting members 24 through springs 22 to have the elasticity in both sides of the signal line 18. Meanwhile, a connecting member 34 connecting the signal line 18 is formed on the bottom of the moving plate 20. A contact member 32 is formed on the connecting member 34 to be projected in a downward direction to contact the signal line 18. The moving plate 20 includes a plurality of small rectangles which are holes formed to eliminate a sacrificial layer.
If a predetermined level of a driving voltage is supplied to the bottom electrode 14 and the top electrode 16, the moving plate 20 moves in a downward direction by the electrostatic force generated between the bottom electrode 14 and the top electrode 16. Accordingly, the connecting member 34 disposed on the bottom of the moving plate 20 connects disconnected portions of the signal line 18 to perform the switching operation. Such a conventional MEMS switch having the membrane structure was disclosed in U.S. Pat. No. 6,307,452.
In the switch having the membrane structure, the signal line 18 and the supporting member 24 are separated with a comparatively long distance. Therefore, the surface of the top electrode 16 may be modified by the thermal expansion while manufacturing the switch or operating the switch. Such a modification of the surface may cause the open problem which permanently opens the moving plate 20 and the signal line 18 not to be contacted. Or, the modification of the surface may cause the stiction problem which narrows the top electrode 16 and the bottom electrode 14 to be connected one another. Such problems degrade the stability and the reliability of the MEMS switch.
If the moving plate 20 and the spring are modified by the thermal expansion, the moving plate 20 cannot maintain to be parallel to the substrate 12 when the moving plate 20 moves. It is because that the supporting member 24 is fixed at the substrate 12 having less thermal expansion rate than the moving plate 20. That is, the moving plate 20 is extremely expanded while the distance between the supporting members 24 is not changed. Such a thermal expansion generates a great stress on the connecting portion between the moving plate 20 and the spring 22, and it modifies the connecting portion, permanently. Finally, the moving plate 20 is abnormally apart from the substrate 12, or the moving palate is titled to one side according to the modification of the moving plate 20 so that the MEMS switch cannot be operated, normally. If the moving plate 20 is lowered to be close to the substrate 12, the connecting member 34 of the moving plate 20 is contacted to the signal line 18, permanently.
Furthermore, the stiction problem is easily occurred because the positive electrode is maintained within an extremely short distance, i.e., several micrometers, to generate the electrostatic force. That is, the moving plate 20 or the spring 22 is easily attached to near fixed other parts. Such a stiction problem is the major factor degrading the reliability of the switch.
As described above, the conventional MEMS switches having the cantilever or the membrane structure have low reliability and low signal isolation characteristics caused by the structural problems such as the thermal expansion and the stiction problem although the conventional MEMS switches are introduced to overcome drawbacks of the conventional semiconductor switches such as high insertion loss, low signal isolation and signal distortion. Therefore, there are great demands for developing a MEMS switch having new structure to overcome such problems.